If your stringers are more than 10’ long, we recommend installing a rigid beam at mid-span to add support to the structure. The strength of stair stringers is considerably weakened by the cuts made to create the stair surfaces, and stairs must be strong enough to withstand heavy foot traffic and very stressful conditions. Both visually and structurally, it is preferable to break long stair runs with intermediate landings. In some cases, however, you may need to use one long run.
A brace will dampen the shear stress that can compromise the integrity of the stringers in the center of the stairs. Most deck stairs are under-built and, as a result, feel bouncy and are at risk to collapse, especially after weathering can propagate cracks in the stringers. You also may consider doubling your stair stringers by nailing two 2x12s together like a beam in order to increase strength. Adding blocking between stringers also can strengthen the entire stair structure.